1. Field of the Invention
The present invention relates to circuitry for a telephone answering device, and specifically to mode switching and voltage regulator circuitry employing operational amplifiers.
2. Description of the Prior Art
In a telephone answering device, the answering cycle is initiated upon detection of a ring signal on the telphone line. This conditions the device to transmit an announcement that is pre-recorded on a magnetic tape loop or other recording medium. At the completion of announcement transmission, the device is switched to the mode wherein the caller's message is recorded. The answering cycle then terminates, and the device is conditioned to answer the next call.
An object of the present invention is to provide an inexpensive telephone answering device that combines low cost with good performance and high reliability. This is obtained by utilizing operational amplifiers that are available as standard commercial integrated circuits to perform the voltage regulation, mode switching and audio amplification functions in the answering device.
For undistorted recording and playback of the announcement and the incoming messages, it is necessary that the recorder drive motor operate at a constant speed. The problem is particularly acute in a battery powered answering device wherein the battery voltage may decrease with time. Use of a regulator to provide constant voltage to the drive motor is imperative.
Prior art voltage regulators usually employ a Zener diode as a reference source against which the motor voltage is compared. An object of the present invention is to provide a voltage regulator that uses an operational amplifier exhibiting an input terminal abrupt threshold characteristic both as the voltage reference and as the driver for a series regulator transistor.
In the past, mode switching in a telephone answering device usually has employed relatively high cost relays or flip-flop circuitry. Another object of the present invention is to implement mode switching by means of a novel bistable circuit employing a single operational amplifier. When power is applied at the beginning of the answering cycle, the output signal from this bistable circuit is used to enable amplification of the outgoing announcement and to disable amplification of audio incoming from the telephone line. When announcement transmission is complete, the bistable circuit is switched to its second stable state in which the output signal now disables the outgoing amplifier and enables amplification of incoming messages.
Advantageously, the outgoing and incoming amplifiers both are implemented by operational amplifiers contained in a single commercially available integrated circuit chip that also contains the operational amplifiers used in the voltage regulator and in the bistable circuit. Low cost and circuit simplicity is achieved. Although described herein in conjunction with a telephone answering device, the bistable circuit and the voltage regulator each are novel circuits that may be used in other applications.